Assessing the effects of ocean diffusivity and climate sensitivity on the rate of global climate change

The range in the projections of future climate warming can be attributed to the inherent uncertainty in the representation of climate model parameters and processes. In this study, we assess the effect of uncertainty in climate sensitivity and ocean heat uptake on the rate of future climate change. We apply a range of values for climate sensitivity and ocean diapycnal diffusivity in an ensemble of simulations using an intermediate-complexity climate model. We further use probability density functions to estimate the likelihood of each model outcome; using this framework, we calculate a range of likely rates of temperature change in response to a given future CO2 emissions scenario. From this analysis, the most probable maximum rate of temperature change lies between 0.3 and 0.5 °C/decade, with a most likely value of 0.36 °C/decade, which is more than twice the observed rate in the late twentieth century. We show that changes in ocean diffusivity have a significant effect on the rate of transient climate change for high values of climate sensitivity, while they have little influence when climate sensitivity is low. The highest rates of warming occur with high values of climate sensitivity and low values of ocean diffusivity. Such high rates of change could adversely affect the adaptive capacity of healthy functional ecosystems.